1. Field of the Invention
The present invention relates generally to agriculture and plant genetic engineering and more specifically to the production of genetically modified vascular plants in which the natural process of lignification is reduced or enhanced.
2. Background Information
Plant cell wall lignins (from the Latin lignum: wood) occur exclusively in higher plants and represent the second most abundant organic compound on the earth's surface after cellulose, accounting for about 25% of plant biomass. Cell wall lignification involves the deposition of phenolic polymers (lignins) on the extracellular polysaccharide matrix. The polymers arise from the oxidative coupling of three cinnamyl alcohols. The main function of lignins is to strengthen the plant vascular body, and the rigidity and structural support provided by lignification are thought to have had an important role in the successful land colonization of plants. In addition, lignins provide mechanical support for stems and leaf blades as well as resistance to diseases, insects, cold temperatures and other biotic and abiotic stresses. Thus, lignification can be a beneficial process.
Although lignins are essential for competitive survival of vascular plants, their resistance to degradation has had a negative impact on certain agricultural and industrial uses of plants. Animals lack the enzymes for degrading the polysaccharides in cell walls and depend on microbial fermentation to break down plant fibers. High lignin concentration and methoxyl content reduce the digestibility of forage crops, such as alfalfa, by cattle, with cattle able to digest only 40-50% of legume fibers and 60-70% of grass fibers. Lignins are believed to limit forage digestibility by interfering with microbial degradation of fiber polysaccharides. However, small decreases in lignin content are predicted to have a significant positive impact on forage digestibility.
High lignin content also is problematic in the wood products industries, which contribute about 4% of the US Gross National Product and are an important component of the global economy. In wood-pulp and paper industries, lignins are undesirable components that must be removed by costly chemical pulping. Most of the lignin found in the space between the fibers and in the secondary wall is removed during the pulping and bleaching process. The chemical treatments necessary to remove lignins generate pollutants. Thus, both the digestibility of forage crops and the pulping properties of trees are adversely effected by high lignin content.
Genetic engineering has great promise for agriculture because it can accelerate traditional breeding programs, cross reproductive barriers and introduce specific, desired traits. Genetic engineering can be particularly advantageous to forestry because traditional methods are hampered by the long generation times of trees. Yet, previous attempts to generate transgenic plants with altered lignin content have targeted biosynthetic enzymes and resulted in undesirable pleiotropic effects.
Thus, there is a need for identifying genes that specifically regulate the lignification process and for methods of genetically modifying cultivated vascular plants to reduce their lignin content. Such methods would allow the more efficient use of plant biomass in animal husbandry where lignin-containing grass and legume crops are used as forage and in the pulp and paper industries. The present invention satisfies this need and provides related advantages as well.